Assuming you're in good health, your erection problems are likely a case of performance anxiety - in other words, being worried about pleasing her, being good in bed, and getting erect. It's very common for young guys to lose their erection the first time (or first several times) that they have sex. It doesn't mean that anything is necessarily wrong with you - though you should get in the habit of having annual wellness exams with your doctor anyway. It likely just means that you're a little nervous about having sex, getting/staying hard and making it all go smoothly.
A few tips:
1. Make foreplay count
Even though you've tried lots of different foreplay, make sure you're engaging in the kinds of foreplay that you truly enjoy and find exciting/arousing. If you're doing something only because you think you should, it's not as likely to help your erection as if you did something together that you really liked/felt excited by.
2. Make sure the condom fits
I'm assuming you're using condoms, right? If so, make sure to use a condom that feels comfortable on your penis. Ill-fitting condoms can decrease sensation to a man's penis or make it feel uncomfortable or restricted. If your condom feels tight or low on sensation, consider Trojan Ecstasy condoms which are roomier along the shaft. Inspiral is also roomier in some places. If the condom is too big and you're worried it will slip off, try a snugger fitting condom from Condomania. Oh, and those "performance enhancing" condoms to help you last longer? You may want to steer clear of those either forever or until you get the hang of sex. They tend to contain numbing agents in order to help decrease sensation and thus help men last longer. However, for a small percentage of men, they can contribute to much less sensation and eventual erection loss.
3. During masturbation and leading up to game day, practice visualisation.
In your mind, walk yourself through the whole experience of kissing your girlfriend, making out for a while, taking off each other's clothes, making out some more, doing the exciting foreplay things you both enjoy, putting a condom on, making out some more and then having intercourse with your hard and reliable erection. It's like psyching yourself up for a game only the game is sex. If you can see it, you can do it! Learn more about visualisation exercises and how they can help sex inThe New Male Sexuality.
4. Try adding a little lube
If you're both virgins, her vaginal entrance may be tight particularly if she is nervous, which can make her muscles tighten up and make her less likely to lubricate on her own. If you're using a condom, ask her to add water-based lube, such as KY Liquid or Astroglide, on top of the condom once it's already on the penis. Having her add lube to your condom-covered penis can enhance sensation and can be a fun, arousing part of foreplay. It can also make penetration easier, which can help guys to maintain their erection.
Yes, it's sex and that can feel like a very big deal at first - but it's also "just sex". It's doable. And you will totally figure it out. It takes most people a little practice and trial and error, like learning any new skill. Once you get the hang of this, you will - in all likelihood - be doing this for much of the rest of your life. So relax, breathe deeply, if you catch yourself trash talking yourself then tell your inner critic to hush, and rev yourself up the way you would for anything important. Tell yourself that you're awesome, she's into you, sex will be great and you can kick back and enjoy. And then enjoy every moment, knowing that it truly does get better and get better.